The empty substrate integrated waveguide (ESIW) technology is recently receiving special attention, since it preserves the many advantages of SIW circuits but provides an enhanced behavior due to avoidance of dielectric filling. Many circuits have been designed in the ESIW technology, including several filters with different performances. The next challenge is to achieve the maximum possible compactness degree for these circuits. In this paper, we present the design of a multilayer empty substrate integrated filter with the same performance as if it were manufactured in a single layer but significantly increasing its compactness and mechanical resistance.

This letter shows the procedure of design of a hybrid directional coupler based on Empty Substrate Integrated Waveguide (ESIW). Once proved in previous works the feasibility of ESIW and the good performance indexes this technology can achieve, the following step is designing useful devices to be integrated in planar circuits. The proposed hybrid coupler, which is the first ESIW 4-port available circuit, is designed at 15 GHz with 3 dB coupling factor and a minimum of 20 dB of isolation over 30% bandwidth. After manufacturing the circuit, simulated and measured results are presented and discussed.

Empty substrate integrated waveguide (ESIW) technology preserves the many advantages of the SIW, such as low cost, low profile, or integration in a printed circuit board. But, since the fields propagate through the air in the ESIW instead of propagating through a lossy dielectric as in the SIW, the ESIW presents lower losses and higher quality factors in resonators. Several transitions have been recently proposed in the literature that connect the ESIW to a classical planar line such as the microstrip. In all these transitions, both the feeding planar lines and the ESIW are built in the same substrate layer (the height of the ESIW cannot be selected independently), and they are based on tapered structures in the planar line and/or in the ESIW that reduce the compactness of the transition. These transitions also combine metallized and nonmetallized parts, which increases the complexity of the manufacturing process. In this article, a new through-wire microstrip-to-ESIW transition is proposed. The feeding lines and the ESIW are implemented in different layers so that the height of the ESIW can be independently chosen, and higher quality factor ESIW filters can be excited. Besides, this novel transition can be manufactured with simple cuts and with uniform metallization so that the manufacturing complexity is reduced. Finally, it is a very compact transition that can be freely rotated; that is, the angle between the feeding lines and the ESIW does not affect the performance of the transition. This provides a high degree of versatility in the design phase.